The Graded-Gap Semiconductor Photoemitter

A new scheme is proposed for achieving photoemission from semiconductors at infrared wavelengths from about 1 micrometers to well beyong 10 micrometers. The device consists of a graded-energy-gap p-type semiconductor with a low work function coating on the wide-gap side. Electrons, which are photoexcited from the valence to the conduction band by long wavelength radiation in the narrow-gap region are drifted by an applied electric field into the wide-gap region from where they are emitted into vacuum. Graded-gap Hg(x-1) Cd(x)Te is considered for a photoemitter covering wavelengths to about 12 micrometers and both Ga(1-x)In(x)As and InAs(x)P(1-x) are proposed as possible alloys for 3 micrometers photoemission. For the simplest case of a linearly graded gap and a constant conductance, expressions are derived for the length of the graded region required to minimize power dissipation and for the speed of response. A calculation is also made of the dark current which results from thermal generation in the narrow-gap region in order to determine the operating temperature required for achieving a desired noise-equivalent power at any wavelength.